1. Field of the Invention
The invention relates to a device for suspending the shadow mask of a cathode ray display tube.
2. Discussion of the Background
Between the display screen provided with luminophores and the electron gun, a color cathode ray display tube comprises a shadow mask consisting of a metal sheet pierced with a multitude of holes. This shadow mask is intended to ensure proper separation of the three electron beams which the electron gun emits, so that each electron beam reaches only the luminophore corresponding to the color associated with it. It is necessary to align the electron beams, the luminophores and the holes in the shadow mask properly in order to obtain a high-quality image.
When the tube is in operation, the shadow mask heats up (its temperature may reach 130.degree. C.), it expands and, in the absence of particular arrangements, this expansion gives rise to misalignment of the electron beams, the luminophores and the holes in the shadow mask. This results in a loss of image quality.
In order to compensate for this unfortunate expansion, the shadow mask is suspended from the glass slab constituting the screen by thermostatic bimetals which generally consist of an austenitic stainless steel and an Fe--Ni alloy whose chemical composition comprises, by weight, from 36% to 42% of nickel. These bimetals, which may be of the "edge welded" type or of the "plated" type, have a linear behavior, that is to say a linear variation of deflection with temperature, at least up to temperatures of the order of 200.degree. C. The bimetals heat up at the same time as the shadow mask and deform in such a way that the distance from the shadow mask to the display screen changes as a function of the temperature of the shadow mask. This results in compensation for the effects generated by the heating of the shadow mask. However, the temperature of the tube itself may increase significantly under the effect of room temperature and reach, for example, 50.degree. C. Under these conditions, the compensation of the heating effect of the shadow mask is no longer sufficient and overcompensation takes place.